1. Field of the Invention
The present invention relates to an apparatus for use in detecting leaks in food packages. In particular, the present invention relates to an apparatus that allows nondestructive leak testing of food packages using trace levels of non-toxic perfluorinated gas as a tracer compound and a detection means employing a region of reduced pressure and laminar air flow wherein a leak may be detected. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SRI8035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Food processors are required by law to assess the integrity of the packages they distribute. Consequently, millions of dollars are spent annually in order to ensure food product safety. Leak tests are necessary to ensure that microbial contamination has not occurred after sealing. Of the greatest concern is the detection of leaks between 25 and 100 microns, or "microleaks." Due to their small size, microleaks are the most difficult to detect and therefore random sampling is required. Given the state of the art, random sampling involves removing the sample from the assembly line and subsequently performing destructive tests that render the product unfit for sale. This destructive testing is inefficient and costly. Furthermore, the results of these tests are statistically limited by the size of the sample.
Current gas leak detection tests for industrial processing involve the injection of a pressurized tracer gas into a hollow section of a container followed by monitoring of the area surrounding the container for the presence of tracer gas. For example, helium leak testing is widely used for industrial applications and has been applied to food packaging on a limited basis. Being nontoxic, odorless and tasteless, helium can be introduced into sample glasses or metal containers and if leaks exist, such leaks can be detected by external detection devices. Gas leak detection is virtually the only method by which a container may remain intact after testing.
Current tracer gas detection is typically carried out using a "sniffer wand", i.e. a long hollow tube attached to an air pump and detector. The wand is passed over the surface of a large container or pipe, or through an array of smaller ones, in order to find the highest concentration of tracer gas. For example, the refrigeration industry has long used a "torch sniffer" to determine the existence and location of Freon.TM. and other chlorine based gas leaks. In this application, a sniffer wand is attached through a flexible hose to the air intake of a blowtorch whose flame contacts a copper plate or mesh. Refrigerant gas passes through the wand and hose assembly and decomposes in the flame. The elemental chlorine reacts with the copper to form a volatile copper chloride which gives the flame a distinctive bluish--green glow. The appearance of this glow thus signals a leak, which can be located by moving the wand until the glow is brightest. Freon.TM., while non-toxic, odorless, tasteless, and stable under most conditions, is not suitable for widespread use in testing food product containers for leaks because its solar radiation induced decomposition in the upper atmosphere poses a danger to the protective ozone layer which blocks this radiation from the Earth's surface.
Unlike chlorine bearing gases, which can be detected in the presence of copper and heat, other gases require more complex detection methods. With helium leak detection, a mass spectrometer must first sample the air, ionize it, and then pass the ions through crossed electric and magnetic fields, thereby detecting helium by its high charge to mass ratio. Another common tracer gas, Sulfur Hexafluoride (SF6), is detected by irradiating the air with low energy beta-particles (electrons) from a metal foil doped with tritium and detecting the resulting current. Since SF6's outer shell is composed of fluorine, the most electronegative element known, the SF6 readily absorbs and holds these electrons, thereby altering the current in the metal foil. This permits the detection of SF6 in air at extremely low concentrations, well below one part per trillion.
While the detection methods used in tandem with helium and SF6 tracer gases are highly sensitive, they are slow to respond. The mass spectrometer utilized in helium testing requires several seconds to process a sample while a typical gas chromatograph requires more than a minute to identify the presence of SF6. This makes any type of sniffer-wand application very time and labor intensive. The problem is compounded when a large number of relatively small, individual containers must be tested for leaks, since a great many separate measurements will be needed to identify which one or few among them is actually leaking. The slow response time of the present methods and the high rates at which food packages are produced and distributed, makes one hundred percent leak testing impossible.
A further problem with gas detection methods arises when the pressure within the container to be tested is equal to or less than that of the external environment; a common occurrence with containers sealed at high temperatures when they are tested after cooling. Under these conditions, no net outward flow of gas from the container will occur even at a leak, and the only escape of tracer gas will be through diffusion. As a result, the identification of leaks is almost impossible. Net outward gas flow can be re-established by placing the container in a vacuum chamber wherein the external pressure is less than the pressure inside the container. However this procedure increases both the cost and time required to detect leaks.
Therefore there exists a present need for testing for leaks in food packages nondestructively and efficiently so that one hundred percent leak detection is possible and cost-effective.